2 Answers
2

So that the ticket you have purchased online can be checked, you must show the printout with a valid ID document.

However, if you have an account with SBB, you can just download the app and access your ticket there (again quoting from above linked page):

It is now also possible to display tickets bought online on the SBB Mobile app and print out tickets purchased via SBB Mobile at SBB.ch. The only precondition for this is that the tickets have been purchased via the same user account.

So that the ticket you have purchased online can be checked, you must show the printout with a valid ID document. Alternatively, you can also show the ticket purchased on the SBB Mobile app.

The latter option is however only available if you bought your ticket on sbb.ch and not on some other travel website.

After downloading the SBB Mobile app and logging in with the same account you used to purchase the ticket online, your ticket should automatically appear on your phone.

Just presenting the PDF on your phone does not seem to be an option, so you will have to print it out if you don't want to install the app or didn't book directly with SBB.

Edit: Referring to Jonathan's comment below: The website, in the FAQ section for online tickets, explains what you should do if you have printing issues (which is also what you should do if you don't want to print out the ticket):

You have 3 options:

Option 1: display the ticket in the SBB Mobile app.

Option 2: you can have your ticket sent via MMS on your mobile phone. However, you can only obtain this so-called MMS ticket for selected routes.

Option 3: rectify printing problems.In order to view and print the ticket, you must have a PDF reader installed on your computer.

That means they really don't like the idea of you showing them the PDF file on your phone (not even as option 4 after "fix your printer"), so it does not seem very advisable. However, I don't have any personal experience about whether it would be accepted in practice.

"Just presenting the PDF on your phone does not seem to be an option." - but does it work in practice?
– JonathanReez♦Apr 12 '17 at 14:27

@JonathanReez I am not sure about that, what I wanted to say is that this method is not mentioned on the website and therefore you shouldn't rely on it being accepted when your ticket is checked.
– MophotlaApr 12 '17 at 14:29